In most computing systems, one or more display monitors are utilized to provide a visual input/output capability. Such display monitors are similar in many respects to conventional television receiver displays. Thus, many technologies, including the present invention, may be applied effectively to both. In both systems, a cathode ray tube (CRT) includes an evacuated envelope usually made of high-strength glass. The envelope includes a generally flat or slightly curved faceplate or viewing screen together with a funnel shaped bell and extending neck. The interior side of the faceplate supports a phosphor screen. In monochrome displays, a single electron gun is supported within the CRT neck and is directed toward the phosphor screen. The electron gun produces a beam of electrons which are directed toward the faceplate striking the phosphor screen and causing visible light to be emitted therefrom. In color display systems, a plurality of electron guns are used together with a phosphor screen which supports plural areas of phosphors having differing color light emitting characteristics. A shadow mask or similar structure is interposed between the electron guns and the phosphor screen to cause each of the electron guns to stimulate an associated type of colored light emitting phosphor.
Whether the display system is monochrome or color, the electrons emanating from the electron gun or guns form a CRT beam which is scanned in both the horizontal and vertical directions across the faceplate to form a raster. In most instances, the horizontal scan system is operative at a higher frequency than the vertical scan system. Thus, the horizontal scan moves the electron beam rapidly from side to side across the faceplate while the vertical scan system causes the successive horizontal scans to be moved progressively from top to bottom to complete a display frame and form the raster.
In the majority of the presently used display systems, electron beam scanning is accomplished by electromagnetic deflection of the CRT beam. A deflection yoke is supported upon the CRT envelope between the electron guns and the faceplate. The deflection yoke supports a plurality of deflection coils which are coupled to the horizontal and vertical scan systems. Horizontal and vertical scan signals provided by the respective scan systems are coupled to the windings of the deflection yoke to produce corresponding electromagnetic fields which bend the electron beam and thereby direct it to the desired portion of the CRT faceplate. Both the horizontal and vertical scan signals include longer duration sloped scan portions followed by shorter duration high amplitude retrace portions. The latter are utilized at the completion of each respective scan interval to return the electron beam to its starting position. In addition, the retrace portion of the horizontal scan signal is used to develop the high voltage necessary to accelerate the electron beam toward the CRT faceplate.
The character of the image displayed in a CRT display system results from variation or modulation of the intensity of the scanned CRT electron beam. This intensity modulation must be properly timed or synchronized to the horizontal and vertical rate scanning of the raster. Thus, as the electron beam is scanned across the faceplate to form a raster, the desired portions of the faceplate are illuminated by synchronized modulation of the electron beam to provide the desired image.
One of the primary objectives of such CRT display systems is the production of a sharp finely detailed display image. Toward this end, practitioners in the display system art exercise great effort directed at providing drive systems for the CRT beam intensity modulation which preserve high frequency resolution and sharp rise and fall times of the intensity modulation signal. In addition, the sharpness or crispness of the displayed image is also determined in part by the performance of the CRT display device itself. One of the important determinants in the quality of image is found in the degree to which the CRT beam is accurately focused upon the phosphor display screen. To properly focus the CRT beam, focus electrodes are supported within the neck portion of the CRT which when energized produce the desired electrostatic field which acts upon the CRT beam to obtain focus. Because the CRT beam source is generally centered within the CRT neck and is generally aligned with the center of the CRT display screen, a reference center focus or "static focus" is obtained by coupling the focus electrodes to a source of DC voltage.
While center or static focus is easily obtained, the geometry of the typical cathode ray tube introduces focus error as the CRT beam is scanned in the horizontal and vertical directions. Most, if not all, cathode ray tubes are fabricated with faceplates having radii of curvature which are substantially greater than the distances from their faceplates to the electron gun apertures of their CRT beam sources. As a result, an inherent focus error is created which generally increases with off-center distance. The more recent introduction of CRT display systems utilizing cathode ray tubes having virtually flat faceplates has further increased the significance of this focus error.
Because of the desire to display a sharp crisp image over the entire faceplate area, practitioners in the art have been led to develop various dynamic focus systems which attempt to adjust CRT beam focus as the CRT beam is scanned across the faceplate. While the detailed structures of such systems vary substantially, most attempt to use the horizontal and vertical scan signals to derive dynamic focus signals having a parabolic character. It has been found that the addition of parabolic focus signals at the horizontal and vertical scan rates to the static or DC focus provides substantial improvement of CRT beam focus.
While the basic task of dynamic focus correction is relatively straightforward to understand, the achievement of quality dynamic focusing in an efficient, cost effective and high performance manner is not. In addition, the task of dynamic focus correction is further exacerbated by cathode ray tube manufacturing tolerance variation as well as certain defocusing effects within the CRT beam deflection fields.
While dynamic focus correction provides substantial improvement of CRT beam focusing over large parts of the scanned display, focusing problems often persist in the corner portions of the CRT display screen. The presently used dynamic focusing systems enjoy their maximum effectiveness at the center-left and center-right portions of the display screen as well as the top-center and bottom-center portions. This results primarily from the manner in which dynamic focusing systems utilize and process horizontal deflection rate and vertical deflection rate signals to provide a composite dynamic focus correction signal. As a result, in most display system manufacturing and setup processes, cathode ray tubes having focusing problems at one or more of the display screen corners are accommodated by exercising some measure of overall focus compromise to compensate for the otherwise uncorrectable problem corner areas. While the careful adjustment of the dynamic focus controls of a conventional focus correction system can often achieve a more or less acceptable result, such compromises can degrade the display system performance.
There arises a need, therefore, to provide a dynamic focusing system for CRT displays which exercises independent control over the focus in the corner areas of the display screen.
Accordingly, it is a general object of the present invention to provide an improved cathode ray tube display system. It is a more particular object of the present invention to provide an improved cathode ray tube display system having improved capability for corner focus correction. It is a still more particular object of the present invention to provide an improved cathode ray tube display focus system which permits independent corner focus adjustment.